This invention relates generally to apparatus for controlling a time-varying electrical signal and more particularly, but not by way of limitation, to apparatus for controlling currents and/or voltages in a multi-phase power system.
In a three-phase power system, for example, to which a three-phase motor is connected for energization, many problems can arise which degrade the system and the operation and life of the motor. For example, when the motor is started, significant electrical and mechanical stresses are imposed on the motor because of the high starting currents normally used to start the motor. For example, there are electric submersible motors which have a starting current of six to eight times the nominal rating. Such high starting currents cause the system voltage to sag and thereby affect not only the motor, but also other electronic circuits on the system.
Even after a motor is started, significant stresses can be applied to the motor due to unbalanced operating currents. These unbalanced currents can cause excessive heating and increased power consumption. Also during operation, electrical transients can occur in the system, such as from lightning or switching surges. These undesirable operating conditions adversely affect not only the electrical load, but also the overall system.
The foregoing illustrates the need for an apparatus which eliminates electrical system imbalances, whether they be current imbalances or voltage imbalances. There is also the need for an apparatus to eliminate or reduce electrical transients. There is the further need for an apparatus which allows the soft-starting of electrical loads, such as motors, to reduce excessive stresses imposed upon such loads during start-up. If these needs were met, the operating life and dependability of electrical loads could be increased and power consumption could be reduced.
An apparatus which meets such needs should be electrically and mechanically dependable and efficient to enhance the structural, operational and economic features of such an apparatus. Such an apparatus should also be capable of being installed and removed from the power system without causing expensive downtime of the system. Such an apparatus should also be capable of use without additional step-up transformers and with a minimum of special training for installation and maintenance.